In an effort to protect vehicle operators and occupants, vehicle manufacturers and their suppliers are increasingly designing and manufacturing vehicles with additional and improved safety features. One such safety feature that has been incorporated into a number of vehicle types is anti-lock braking, which can take a variety of forms.
Ideally, the braking forces at the wheels increase proportionately as the driver increases the force on the brake pedal. Unfortunately, this is not always the case in braking procedures. As the braking torque and hence the braking force at the wheel is increased, the rotational speed of the braked wheels becomes less than the speed of the vehicle. When the rotational speed of a wheel is less than the vehicle speed, "slippage" is said to occur between the tire and the surface. With further increase in brake pressure, the slippage between the tire and the surface increases until lockup and skidding of the wheel occurs. In most cases, lockup causes a reduction in braking force and an increase in stopping distance. Lockup also causes a degradation in directional control due to a reduction in the lateral forces at the wheels.
Both of these problems associated with lockup were addressed with anti-lock brake systems (ABS). A basic ABS monitors the velocity at each of the wheels, decides whether the wheel is excessively slipping based on these velocity measurements, and modulates the braking pressure accordingly to avoid lockup. The ABS aids in retaining vehicle stability and steerability while providing shorter stopping distances.
One method by which a state of excessive slippage is identified in the ABS involves comparing the speed of each wheel to a reference speed to determine if a slip threshold has been exceeded. The reference speed is an estimate of the true vehicle speed based on current and previous values of the individual wheel speeds. The slip threshold is a predetermined departure of wheel speed from a preselected speed. If the speed of a wheel is significantly less than the reference speed, or exceeds the slip threshold, then the wheel is deemed by the ABS to be excessively slipping. The ABS then reduces the pressure actuating the brake in order to reduce brake torque. The reduction of brake torque allows the friction force at the surface to accelerate the wheel, thereby causing a reduction of the slip in the wheel.
After a period of constant braking pressure following the pressure reduction, the pressure actuating the brake is increased until excessive wheel slip occurs again. The cycle of decreasing the brake pressure, maintaining constant brake pressure, and then increasing brake pressure is repeated until the anti-lock event ends. The parameters which define the specifics of this cycle depend on the operating conditions.
If a wheel has significant acceleration when recovering from a departure, the wheel speed will likely overshoot the vehicle speed reference and oscillate for a period of time. Therefore, the need exists within an anti-lock brake system for damping wheel speed oscillation in order to maintain good control of the wheel.